<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <script src="https://cdn.staticfile.org/react/16.4.0/umd/react.development.js"></script>
  <script src="https://cdn.staticfile.org/react-dom/16.4.0/umd/react-dom.development.js"></script>
  <script src="https://cdn.staticfile.org/babel-standalone/6.26.0/babel.min.js"></script>
  <title>Document</title>
</head>

<body>
  <div id="render"></div>
  <script type="text/babel">
    class Clock extends React.Component {
      constructor() {
        super()
        this.state = { age: 18 }
        this.addAge = this.addAge.bind(this)
        this.cutAge = this.cutAge.bind(this)
      }
      addAge() {
        if (this.state.age < 25) {
          this.setState(res => ({
            age: ++res.age
          }))
        } else {
          this.setState({
            age: 18
          })
        }
      }
      cutAge() {
        if (this.state.age > 0) {
          this.setState(res => ({
            age: --res.age
          }))
        } else {
          this.setState({
            age: 18
          })
        }
      }
      render() {
        return (
          <div>
            <h1>{this.state.age}</h1>
            <button onClick={this.addAge}>快点长大</button>
            <button onClick={this.cutAge}>不想长大</button>
          </div>
        )
      }

    }
    ReactDOM.render(
      <Clock />,
      document.getElementById("render")
    )
  </script>

</body>

</html>